Seasonal Dependence of the Magnetospheric Drag Torque on Saturn's Northern and Southern Polar Thermospheres and its Relation to the Periods of Planetary Period Oscillations. Issue 9 (11th September 2020)

• Record Type: Journal Article
• Authors: Cowley, S. W. H.; Achilleos, N.; Bradley, T. J.; Provan, G.; Bunce, E. J.; Hunt, G. J.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 9(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-09-11
• Subjects: auroral region -- ionosphere -- magnetosphere -- planetary period oscillations -- Saturn; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2020JA028247
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: We calculate the magnetospheric drag torques on Saturn's northern and southern polar thermospheres during late southern summer in 2008 and northern spring in 2012–2013 using previously derived profiles of ionospheric meridional coupling currents determined from high‐latitude Cassini magnetic field data. We show that the drag torques in the "winter" and "summer" auroral regions are near equal at ~2.3 × 10 16 N m, contrary to the recent discussion of Brooks et al. (2019, https://doi.org/10.1002/2019JA026870 ) who suggest that significant seasonal differences should occur in these regions. Instead, seasonally dependent torques occur in the adjacent polar open field regions, where the "winter" and "summer" torques are ~0.3 × 10 16 and ~1.8 × 10 16 N m, respectively. We derive a simple rotating disc model of the polar thermosphere and estimate the speed of the poleward flow from midlatitudes required to balance these torques in steady state, finding values of tens of m s −1 consistent with previous numerical modeling. Comparison of the calculated torques with concurrent periods of the northern and southern planetary period oscillations (PPOs) does not suggest a direct connection between these quantities as proposed by Brooks et al., 2019, showing at the least that significant additional factors must be involved. We further note some issues with their scenario for dual modulation of radio emissions, previous observations having shown that the principal oscillatory PPO field‐aligned currents that modulate the emissions rotate in the auroral region with periods ~10.7 ± 0.1 hr, propagating through the more slowly rotating ~15–20 hr period outer magnetospheric plasma, with implications for the proposed "atmospheric flywheel" picture. Key Points: We calculate the magnetospheric drag torque on Saturn's polar thermosphere across equinox using current profiles derived from Cassini data Contrary to recent suggestion major seasonal variations are confined to the polar open field region and do not occur in the auroral region Comparison of the torques with the periods of planetary period oscillations does not suggest a direct connection between these quantities